Why doesn't the type of the return value satisfy Partial<T>?

Here is the code:

type User = {
  id: number;
  kind: string
}
function m<T extends User>(u: T): Partial<T> {
  return {id: 1}
}

Since {id:1} is of type {id:number}, which is a subset of T, the code should work normally. But it throws an error saying that "Type '{ id: 1; }' is not assignable to type 'Partial<T>'."

I can't figure out why the type infered result of {id:1} is {id:1} itself instead of {id:number}. But anyway, I can assert that it actually is of type {id:number}, like this:

type User = {
  id: number;
  kind: string
}
function m<T extends User>(u: T): Partial<T> {
  return {id: 1} as {id: number}
}

But still, it again throws an new error saying that "Type '{ id: number; }' is not assignable to type 'Partial<T>'". Strangely, isn't {id: number} a subset of T?

So here I have several questions:

1. Why the type infered result of {id:1} is {id:1} itself instead of {id:number}?

2. Why it still throws an error even if I have asserted that {id:1} is of type {id: number}?

3. How can I make the code work normally?